The invention relates to a projection television display tube having an evacuated envelope with a display window which on its inside comprises a display screen and in front of which a light permeable second window is provided on its outside, a cooling liquid flowing through the space between the display window and the second window from at least one inlet aperture to at least one outlet aperture, namely by temperature differences in said cooling liquid.
The invention also relates to a picture display device having one or three such projection television display tubes.
Such a display tube is known from U.S. Pat. No. 4,529,905, P. Koshictal. By means of an electron beam a field is written on the display screen which usually comprises a phosphor layer or a pattern of different phosphors. As a result of the electron bombardment the temperature of the phosphor increases so that the luminous efficiency of the display screen decreases ("thermal quenching"). This phenomenom occurs in particular in display tubes for projection television in which for maintaining the required high luminous densities the display screen is scanned by electron beams of high beam currents. At the same time the temperature of the display window increases and a temperature gradient is formed at the display window. This gradient causes a mechanical stress in the display window which consists, for example, of glass. At high electron beam current and consequently high thermal load, this may lead to fracture of the display window. In order to reduce said mechanical stresses in the display window by temperature differences ("thermal stress") and to avoid the reduction of the luminous efficiency it is known from the already mentioned U.S. Pat. No. 4,529,905 to cool the display window and the display screen connected thereto. The space between the display window and the second window filled with cooling liquid in a first described embodiment is surrounded on the top, at the bottom and, laterally by a metal cooling member which serves as a spacing member and as a heat radiator. As a result of the rise in temperature of the display window the cooling liquid heated by the display window moves along the display window upwards and past the second window downwards as a result of which the thermal energy from the centre of the display window is also dissipated via the cooling member. At low load, for example smaller than 5 W, the thermal energy is dissipated to the second window substantially by conduction. At higher load the above-described liquid flow occurs with an associated additional cooling by the cooling member, which however, is little effective. Moreover, an embodiment is described in which the cooling liquid is applied to the space from the top side of the space through pipes or hoses and through a cooling chamber to the lower side, namely by flow caused by temperature differences in the cooling liquid. A disadvantage of such a tube is that when the tube in a projector has to be replaced the cooling liquid must be removed and the hoses and pipes, respectively, must be disconnected from the display tube.
A similar liquid cooling is suggested in U.S. Pat. No. 4,734,613 in which a laminar flow of the cooling liquid along the display window is used at a flow rate of approximately 5 cm.sup.3 /s. With this type of cooling a power up to 60-80 W can be dissipated. The backflow of the cooling liquid from the outlet aperture to the inlet aperture of the space occurs through hoses or pipes and by means of a pump provided in said system of hoses and pipes. In said system of hoses or pipes a cooling chamber (heat exchanger) is also incorporated. In such tubes in which an accelerating voltage of 29 kV is used, 60-80 W corresponds to an average beam current of 2-3 mA with peak currents up to approximately 10 mA.